The invention relates to chemical etching, and more particularly to a method of forming the art work for chemically etching that produces uniform through-etch and lateral-etch.
Etching of metal sheets to form lead frames and other metallic articles with closely spaced members is accomplished by defining apertures in a photo resist material on either one or both sides of a metal sheet. Chemical etching is performed through the apertures to remove metal from the metal sheet to form a desired pattern or design. The chemical etching may be from one or both sides of the metal sheet. When chemical etching is done from both sides, the openings in the photo resist material coating the metal sheet are positioned so that the openings on each side of the metal sheet are aligned with each other. The chemical milling or etching of intricate patterns found in semiconductor lead frames is extremely dependent upon precise and accurate photo mask artwork.
In the etching process, it would be desirable if the etching chemical would only etch vertically through the metal sheet. However, horizontal etching occurs during the etch process. To allow for the horizontal etching, known as undercutting, the photo resist mask is designed to compensate for the undercutting. The degree to which lateral etching, or undercutting, occurs in relation to veritcal etching is influenced in part by the size and geometry of the individual features of the pattern to be etched.
Typical practice is to determine the varying etch rates, both through-etch and lateral-etch, through trial and error, or utilize data bases established from experience to design the photo mask artwork corresponding to a desired part design.
The formation of small dimensioned apertures for photo chemical masking and etching is described in U.S. Pat. No. 3,653,898. Other photo mask processing is described in U.S. Pat. Nos. 5,426,007 and 3,942,981.
The invention is a method of forming the art work for chemically etching that produces uniform through-etch and lateral-etch. The artwork that defines the pattern to be etched utilizes lines equal to the narrowest feature that is to be etched. Rather than etch away large areas, section are removed by etching by cutting them out of the material that is being etched. The artwork or pattern is designed with the same compensation factors throughout the entire pattern and the etch rate will be completely uniform for the entire pattern.